When using polymeric insulating material with a high specific resistance, for example polyethylene, space charges may form within the material. These space charges distort the electric field in the insulating material and this causes electrical breakdown at a lower average field strength than without these space charges. This problem becomes especially pronounced for electrical insulation which is subjected to high direct voltage, in particular cables for transmission of high-voltage direct current.
To ascertain the generation and distribution of the space charge within the insulating material, a need has arisen to provide a non-destructive method which may provide a picture of the space-charge distribution within the insulating material in a manner similar to that in which, for example, the emission of X-rays or ultrasonics is used to demonstrate variations in the material composition of a test specimen.
Two methods which may be used for non-destructive space-charge measurement are the pressure-wave propagation method, PWP, and the pulsed electroacoustic method, PEA.
In the PWP method, the material is subjected to a mechanical pulse, which generates an acoustic wave which during the propagation "shakes" space charges. This generates a current signal between short-circuited electrodes or a voltage signal over open electrodes which surround the insulating material, which signal is then evaluated with respect to the space-charge distribution.
In the PEA method, the material is subjected to a short electric voltage pulse between two electrodes which surround the insulating material, and the force exerted by the thus caused electric field on the space charge "shakes" the insulating material. This mechanical displacement propagates through the material as an acoustic wave which is detected, for example with a piezoelectric transducer, and is then evaluated with respect to the space-charge distribution.
A good summary of the prior art relating to the PEA method is given in the publication by N. Hozumi, T. Okamoto and T. Imajo: "Space charge distribution measurement in a long size XLPE cable using the pulsed electroacoustic method", Proc. of IEEE Symposium on EL. Ins. 1992, pp. 294-297, Baltimore, Jun. 7-10, 1992.
The original method makes use of parallel plates as electrodes with the insulating material to be tested between these. One of the electrodes is in mechanical contact with a plate of piezoelectric material which converts the acoustic signal into an electrical signal. When changing to a cylindrical geometry, with an inner electrode which is concentrically surrounded by insulating material, which in turn is surrounded by an outer conducting layer, short pieces of cable may be tested using the PEA method. The inner electrode consists of the inner conductor of the cable, the outer electrode of a sector-shaped aluminium block in mechanical contact with the semiconducting layer which is applied onto the cable insulation. The aluminium block is mechanically connected to a piezoelectric PVDF film which, in a manner previously described, generates an electrical signal when hit by the acoustic wave.
To achieve a sufficiently high resolution during the space-charge measurement, the pulse length of the electric pulse must be short compared with the time needed by a acoustic pulse to propagate through the insulating material over the electrode distance. This entails pulses with a duration of a few 10 ns, in which case the frequency contents of the pulse will primarily be within the range of 10-100 MHz. Since the space-charge measurement is to be carried out during simultaneous application of a high direct voltage to the inner conductor of the cable, both the coupling capacitor for the pulse application and the end termination of the cable become large in their linear dimensions and therefore constitute a relatively great inductance, which greatly influences the pulse shape. To be able to carry out space-charge measurements on longer cable pieces, the outer conductor (metal foil or screen cover) of the cable is removed at the location where the cable is to be tested. In the middle of this zone, without an outer conductor, an annular outer electrode of metal foil is applied, in mechanical contact with an aluminium block as above. The semiconducting layer, which in this context has a high electric resistance, is left intact. The voltage pulse is applied between the annular outer electrode and the outer conductor of the cable and is connected capacitively to the inner conductor through the impedance of the cable. The fact that part of the pulse makes its way through the semiconducting layer reduces its voltage but has otherwise no disturbing influence.
The above method solves the problem of measuring on a long cable. However, this is done at the cost of a new problem:
During the application of the pulse, the annular outer electrode is energized. This pulse voltage pulse must not reach measurement and evaluation equipment located at ground potential. One way of solving the problem is to apply an electrically insulating layer, for example a plastic foil, between the outer electrode and the aluminium block. However, the foil disturbs the propagation of the acoustic signal which now has to pass through the insulating layer. It may also be charged and thereby generate disturbance. Another method, also suggested in the publication, is to allow the measurement equipment to be in electrical contact with the outer electrode and to transmit the measurement signals from there in a potential-free manner with the aid of optical fibres to the evaluation equipment. However, this solution is complicated, resource-demanding and more sensitive to disturbance.